Thermoplastic films are known in the art and can be manufactured from semicrystalline resins such as polyethylene terephthalate (PET), polyethylene, cycloolefin copolymers, ethylene vinyl alcohol copolymers, nylons, polylactic acids, polypropylene, polyphenylene sulphate, and other materials. Moreover, it is appreciated that a number of thin polymer films can be prepared commercially with processes that involve sequential melting, extrusion, and solidification of the polymer material into a thin cross-section. Orientation of such polymer films through blowing or mechanical methods in the longitudinal (machine) and transverse directions can significantly increase the crystallinity of the films, which can, in turn, affect tear, twist, optical, and other mechanical properties of films.
In this regard, it is further appreciated that the amorphous content of any PET film is the weight fraction of material that is not crystallized within the biaxial orientation process, and that the densification of the amorphous phase implies that the mass per unit volume of non-crystalline material is increased (see, e.g., Polymer Bulletin, April 1988, Volume 19, Issue 4, pp. 397-401, the entire contents of which are incorporated herein by this reference). The semicrystalline nature of oriented polymer films implies a weight fraction of such film is crystalline, whereas the remaining portion of the material is amorphous. The ratio of crystalline to amorphous domains is strongly influential for properties like gas barrier properties, thermal resistance, density, optical clarity, and other commercially-valuable properties.
With further respect to the structure of oriented polymer films, including PET films, some have suggested that the nodular structure and small isometric crystallites within biaxially-oriented PET film may contribute to PET film ductility at low temperatures (see Klement and Geil, Journal of Macromolecular Science, Part B: Physics Volume 5, Issue 3, 1971, the entire contents of which are incorporated herein by this reference). Such morphological attributes can be introduced and controlled through process conditions including stretching ratios, speed of stretching, temperature of stretching, crystallization temperatures, and the like.
U.S. Pat. Nos. 4,786,533; 4,842,187; 5,292,563; 5,451,455; and 5,573,723 as well as European Patent No. 0441027, which are incorporated herein by reference, describe various formulations and attributes for polyolefin twistable films, where, in general, the modulus of the film is lowered to allow deformation of the film under relatively low stress conditions. More specifically, these patents describe the use of additives to change the modulus properties of biaxially-oriented polyolefin films, as a lower modulus allows the films to twist with little recovery, as well as tear more easily.
Despite the properties observed with those films, however, the construction of films with low modulus properties presents many challenges in the commercial environment. Maintaining low modulus from the inclusion of low molecular weight additives like oils, tackifiers, and plasticizers can result in problems of migration of the low molecular weight additives, which can then result in additional issues related to obtaining food contact approval status and to processing troubles. Surface properties like coefficient of friction (COF) and heat sealability can also be compromised as a function of time in films that include such additives. Furthermore, it can be difficult or impossible to metalize low modulus films of this type of construction due to problems associated with the relatively high vapor pressures of oils and other small molecules under vacuum metalizing chamber conditions.
Other known methods for providing thermoplastic materials with good twist or tear properties have involved making such films at relatively low orientation ratios. However, the construction of low stretch ratio films can be problematic due to the likelihood of high shrinkage during subsequent use of the films as well as poor commercial runability of the films. Such shrinkage would be further exacerbated as temperatures increased.
Accordingly, there remains a need in the art to produce a thermoplastic PET film with easy twist and tear properties without the requirement of using low molecular weight additives or low stretch ratios to construct the material. Such films and processes would be desirable and beneficial for a range of commercial applications including, but not limited to, the packaging of food, candy, novelties, and other commodities.